This application is based on patent application Hei.11-259165 filed in Japan, the contents of which are hereby incorporated by references.
1. Field of the Invention
This invention relates to a driving apparatus and a driving method of an impact type piezoelectric actuator.
2. Description of the Related Art
In an impact type piezoelectric actuator, a driven member is movably connected to a rod shaped driving member in an axial direction thereof and a piezoelectric device is connected on an end of the driving member in a manner so that the polarization of the piezoelectric device coincides with the axial direction. For example, Publication Gazette of Japanese Patent Application Hei.7-298656 shows a conventional impact type piezoelectric actuator used for moving a taking lens of a camera.,
A basic configuration of the conventional impact type piezoelectric actuator is shown in FIG. 14. The piezoelectric actuator 100 comprises a rod shaped driving member 101, a driven member 102, a lamination type piezoelectric device 103 and a driving circuit 104. The driven member 102 is coupled with the driving member 101 by a predetermined friction force. When a force larger than the friction force is applied to the driven member 102, it can be moved in an axial direction of the driving member 101. An object to be moved such as a taking lens is connected to the driven member 102. The piezoelectric device 103 is connected on a base end 101b of the driving member 101 in a manner so that the polarization of the piezoelectric device 103 coincides with the axial direction of the driving member 101. A pair of electrodes 103a and 103b are provided on both ends of the piezoelectric device 103. The electrode 103b is grounded and the other electrode 103a is connected to the driving circuit 104.
The driving circuit 104 is configured by a forward driving circuit 105 for moving the driven member 102 toward a top end 101a of the driving member 101 (hereinafter, this direction is called xe2x80x9cforwardxe2x80x9d), a backward driving circuit 106 for moving the driven member 102 toward the base end 101b of the driving member 101 (hereinafter, this direction is called xe2x80x9cbackwardxe2x80x9d) and a controller 107 for controlling the forward driving circuit 105 and the backward driving circuit 106.
The forward driving circuit 105 is configured by a slow charger 105a and a quick discharger 105b. The backward driving circuit 106 is configured by a quick charger 106a and a slow discharger 106b. The slow charger 105a and the quick charger 106a respect apply a driving voltage Vp to the piezoelectric device 103 in the polarization direction (or charge the piezoelectric device 103 in the polarization direction) for expanding the piezoelectric device 103 in the polarization direction (or in the axial direction of the driving device 101). The quick discharger 105b and the slow discharger 106b respectively apply a potential to the piezoelectric device 103 in the opposite direction to the polarization or discharging the electric charge in the piezoelectric device 103 by grounding the electrode 103a for contracting the piezoelectric device 103.
The slow charger 105a is configured by a constant current circuit for controlling the charging speed by restricting the charging current. The slow charger 106b is configured by a constant current circuit for controlling the discharging speed by restricting the discharging current.
The controller 107 alternately switches the slow charger 105a and the quick discharger 105b in the forward driving operation. The controller 107 alternately switches the quick charger 106a and the slow discharger 106b in the backward driving operation.
When the slow charger 105a and the quick discharger 105b are alternately driven in the forward driving operation, the piezoelectric device 103 repeats the slow expansion and the quick contraction alternately. By such operation, the driving member 101 repeats the forward slow movement and the backward quick movement. When the quick charger 106a and the slow discharger 106b are alternately driven in the backward driving operation, the piezoelectric device 103 repeats the quick expansion and the slow contraction alternately. By such operation, the driving member 101 repeats the forward quick movement and the backward slow movement.
The friction force acting between the driving member 101 and the driven member 102 becomes smaller when the driving member 101 moves quickly, and it becomes larger when the driving member 101 moves slowly. Thus, the driven member 102 moves with the driving member 101 only when the driving member 101 moves slowly. Consequently, the driven member 102 moves forward relative to the driving member 101 in the forward driving operation, and the driven member 102 moves backward relative to the driving member 101 in the backward driving operation.
The impact type piezoelectric actuator is generally used for driving an optical system of a portable equipment such as a taking lens of a camera and a lens of a binoculars. It is preferable to make the driving circuit of the actuator compact and simple with regard to the downsizing and the lightening of the equipment. Since the electric power supply of the portable equipment is a dry battery, it is preferable to drive the driving circuit of the actuator by a voltage of the battery.
The voltage of the battery, however, is relatively lower, so that the quantity of the expansion and contraction of the piezoelectric device 103 becomes smaller when the piezoelectric device 103 is charged and discharged between the voltage Vp of the battery and the grounded voltage (0V). Consequently, the moving speed of the driven member 102 becomes slower. When the moving distance of the driven member 102 is relatively longer, the driving time period of the piezoelectric device 103 becomes longer, and the exhaustion of the battery becomes larger. Furthermore, when the voltage of the battery is dropped due to the exhaustion of the battery or the variation of the ambient temperature, the moving speed of the driven member 102 becomes much slower or the driven member 102 becomes immovable.
For solving the above-mentioned problem, it is proposed to apply a technique for increasing amplitude of output signals virtually double by using the BTL (Bridged Tied Load) connection into the driving circuit 104 of the piezoelectric actuator. The BTL connection is generally used in a car audio system for increasing output power of speaker. By applying the technique into the driving circuit 104 of the piezoelectric actuator, the voltages +Vp and xe2x88x92Vp are alternately applied to the piezoelectric device 103. The piezoelectric device 103 is repeatedly charged and discharged between the voltage +Vp and xe2x88x92Vp, so that the voltage of the driving signal of the piezoelectric device 103 becomes substantially double. The voltage of the electric power supply, however, is applied to the piezoelectric device 103 in the discharging time period in the opposite direction, so that the electric power consumption of the piezoelectric actuator becomes larger than the conventional driving method. When the battery is used as the electric power supply, the exhaustion of the battery becomes faster.
An object of this invention is to provide a driving apparatus for the piezoelectric actuator which can effectively be driven by a battery and can save the electric power consumption without causing the upsizing of the circuit and the reduction of the driving efficiency.
A driving apparatus of impact type piezoelectric actuator in accordance with an aspect of this invention comprises a controller for switching a first driving mode and a second driving mode of the piezoelectric actuator corresponding to a condition with respect to a moving speed of a driven member driven by the piezoelectric actuator.
When the moving speed of the driven member is supposed to be slower than a predetermined threshold speed, the piezoelectric actuator is driven in the first driving mode so that a driving voltage of an electric power supply is applied to a piezoelectric device alternately in positive direction and negative direction by switching a polarity of the driving voltage. Thus, the piezoelectric device repeats expansion and contraction, and a driven member can be moved at a speed faster than the supposed moving speed in a predetermined direction. The moving speed of the driven member can be made faster.
Alternatively, when the moving speed of the driven member is supposed to be equal to or faster than a predetermined threshold speed, the piezoelectric actuator is driven in the second driving mode so that the piezoelectric device is alternately applied the driving voltage and grounded. Thus, the piezoelectric device repeats expansion and contraction, and the driven member can be moved at a speed slower than the supposed moving speed in a predetermined direction and electric power consumption of the battery can be saved.
The condition with respect to the moving speed of the driven member is, for example, at least one of a voltage of a battery, an ambient temperature of an equipment using the piezoelectric actuator, an actual moving speed of the driven member, and so on.